As an apparatus for performing observation of a surface, measurement of surface roughness, etc., of a metal, a semiconductor, a ceramic, a synthetic resin or the like, a scanning probe microscope (SPM=Scanning Probe Microscope) exemplified by an atomic force microscope (AFM=Atomic Force Microscope) for measuring atomic force acting between a probe and a sample surface is widely known. In an atomic force microscope, several measurement modes are used. Recently, a method called a non-contact mode or a dynamic mode is often used in which a cantilever provided with a probe is caused to be vibrated at a resonance point or the vicinity thereof and the interaction acting between the probe and a sample surface in the vibrating state is converted into changes in amplitude, phase, or frequency of vibration of the cantilever and detected.
FIG. 7 is a configuration diagram of a main part of a commonly-used scanning probe microscope. A sample 1 which is an observation target is held on a sample table 2 provided on a substantially cylindrical scanner 3. The scanner 3 includes an XY-scanner 31 that scans the sample 1 in 2-axis directions of X and Y which are mutually orthogonal and a Z-scanner 32 that finely moves the sample 1 in a Z-axis direction orthogonal to the X-axis and the Y-axis. In each scanner, a piezoelectric element that causes displacement by a voltage applied from the outside is used as a driving source. A cantilever 4 equipped with a probe 5 at its tip is arranged above the sample 1, and this cantilever 4 is vibrated by an excitation unit including a piezoelectric element (not illustrated).
In order to detect the displacement of the cantilever 4 in the Z-axis direction, an optical displacement detector 6 including a laser light source 61, a half mirror 63, a mirror 64, and a light detector 65 is provided above the cantilever 4. In the optical displacement detector 6, laser light emitted from the laser light source 61 is substantially perpendicularly reflected by the half mirror 63 and then irradiated to the reflection surface 4a provided on the rear surface of the tip portion of the cantilever 4. The light reflected by the reflection surface 4a of this cantilever 4 is incident to the light detector 65 via the mirror 64. The light detector 65 is, for example, a four-division light detector having a light-receiving surface divided into four in the Z-axis direction and the Y-axis direction. When the cantilever 4 is displaced in the Z-axis direction, the ratio of the amount of light incident to the plurality of light receiving surfaces changes. The amount of displacement of the cantilever 4 can be calculated by performing calculation processing of the detection signal corresponding to the plurality of received light quantity.
The measurement operation of the scanning probe microscope of the aforementioned configuration in the non-contact mode will be briefly described.
By an excitation unit (not illustrated), the cantilever 4 is vibrated in the Z-axis direction at the resonance point or the vicinity thereof. At this time, when an attractive force or a repulsive force acts between the probe 5 and the surface of the sample 1, the vibration amplitude of the cantilever 4 changes. A minute change amount of the vibration amplitude is detected by a detection signal at the light detector 65, and the piezoelectric element of the Z-scanner 32 is feedback-controlled to move the sample 1 in the Z-axis direction so as to set the change amount to zero, that is, keep the vibration amplitude constant. When the sample 1 is scanned in the X-Y plane by controlling the piezoelectric element of the XY-scanner 31 in such a state, the aforementioned feedback control amount with respect to the Z-axis direction reflects minute unevenness on the surface of the sample 1. Therefore, a data processing unit (not illustrated) creates a surface image of the sample 1 using a signal indicating this feedback control amount.
In such a scanning probe microscope, it is configured such that the position adjustment of the laser light source 61 and that of the light detector 65 are respectively performed so that the strongest laser light reflected by the reflection surface 4a of the cantilever 4 is incident to the center of the four-division light receiving surface of the light detector 65 in a state in which there is no deflection in the cantilever 4. Such adjustment in the scanning probe microscope is called “optical axis adjustment” (see, e.g., Patent Documents 1 and 2).
The procedure of a conventional general optical axis adjustment is as follows. That is, first, an image in which the tip end portion of the cantilever 4 or the vicinity thereof is captured from directly above with a video camera 8 capable of performing optical microscopic observation is displayed on the screen of the display unit 9. The left side of FIG. 8 illustrates an ideal captured image (a) at the time of the optical axis adjustment. While confirming this image, an operator performs a predetermined operation with an operation unit 7 so that the laser light spot image 6a comes to an appropriate position at the tip of the cantilever 4 to adjust the position of the laser light source 61 with a drive mechanism 62. As described in Patent Document 1, it is preferable that laser light is projected on a piece of paper placed in front of the light detector 65 and the position of the laser light source 61 is finely adjusted so that the laser light appears most brightly in the projected image. After determining the position of the laser light source 61, the position of the light detector 65 is adjusted so that the spot of the laser light reflected by the cantilever 4 is located at the center of the four-division light receiving surface of the light detector 65.
As described above, in general, the optical axis adjustment in the scanning probe microscope is performed manually by an operator while visually confirming the image captured for the optical axis adjustment. However, as also pointed out in Patent Document 1, since the luminance of the laser light spot is considerably high, as one example is shown on the right side of FIG. 8, in an image (b) captured so that both the laser light spot image 6a and the cantilever 4 fit in the image, the portion including the cantilever 4 becomes considerably dark. It is difficult for the operator to properly grasp the position of the cantilever 4 from such an image. This is one of factors making the operation of the optical axis adjustment difficult.
In order to solve such a problem, in Patent Document 1, a marker indicating the position of the cantilever and a marker indicating the brightness centroid position of the laser light are displayed on an image so that the optical axis adjustment can be performed using these markers. Of course, even with such a method, the optical axis adjustment can be performed, but for an operator familiar with a conventional optical axis adjustment operation, the position adjustment using markers may sometimes be difficult to understand intuitively. For this reason, there is also a strong demand to perform the position adjustment while observing not a marker displayed on an image but an actually captured object image.